This invention is in the field of minority carrier semiconductor devices. The invention relates in particular to methods for improving the operating stability of light emitting semiconductor devices by the intentional introduction of impurities and the devices created using these methods.
Degradation of semiconductor minority carrier devices typically involves an increase in the non-radiative recombination efficiency of the device during the device's operation. The causes of this degradation depend on the type of device, its structure, materials, and operating conditions.
A known double heterostructure light emitting diode ("LED") is shown in FIG. 1. LED 10 is comprised of an optically transparent GaP window/current spreading/contact layer 12, high bandgap AlInP upper confining/injection layer 14, a lower bandgap (Al.sub.X Ga.sub.1-X).sub.0.5 In.sub.0.5 P active layer 16, high bandgap AlInP lower confining/injection layer 18, and conductive substrate 20, which may be formed from GaAs or GaP. P-type contact 21 and n-type contact 23 complete the LED. Light extraction occurs through both the top surface and sides of the LED. The layers are generally doped so that the p-n junction is located near or within the active layer, and ohmic contacts 21 and 23 are made to the p-type and n-type regions of the device. The structure may be grown by any of a variety of methods including metal-organic chemical vapor deposition("MOCVD"), vapor phase epitaxy("VPE"), liquid phase epitaxy("LEP"), molecular beam epitaxy("MBE"), and others.
FIG. 2 is an energy band diagram of the LED shown in FIG. 1. When forward biased, efficient injection of the minority carriers into active layer 16 is achieved by careful placement of the p-n junction. The minority carriers are confined within the active layer of the LED by high bandgap confining layers 14 and 18. The recombination process consists of both radiative recombination which produces the desired light emission and non-radiative recombination, which does not produce light. Non-radiative recombination may result from crystal imperfections within the LED as well as other causes. Light is extracted from the LED through the LED's various transparent layers and surfaces and is focussed into a usable pattern by various reflectors and lenses(not shown).
The LED illustrated in FIG. 1 is only one example of a minority carrier device. A variety of other minority carrier devices, including bipolar transistors, photodetectors, and solar cells operate on similar physical principles. Semiconductor lasers often have a double heterostructure and similarly experience the competition between radiative and non-radiative recombination. The performance and stability of all these devices depends upon maintaining a long carrier recombination lifetime throughout the operating life of the device.
For the LED of FIG. 1, the output power is directly proportional to the internal quantum efficiency and can be expressed as: EQU .eta..sub.external .varies..eta..sub.internal .varies.1+(.tau..sub.r /.tau..sub.nr)!.sup.-1,
where .eta..sub.internal is the internal quantum efficiency, .eta..sub.external is the external quantum efficiency, .tau..sub.r is the radiative recombination lifetime, and .tau..sub.nr is the non-radiative recombination lifetime. .tau..sub.nr is inversely related to the number of non-radiative recombination centers in the active region. The relationship .eta..sub.external and the concentration of non-radiative recombination centers is illustrated by the graph shown in FIG. 3, which shows the external quantum efficiency .eta..sub.external decreasing as the concentration of non-radiative recombination centers increases. A variety of crystal defects can act as non-radiative recombination centers, including substitutional or interstitial impurities such as Cr, Cu, Au, Fe, O and even such shallow dopants as Si, S, Se, native point defects such as self-interstitials and vacancies, impurity or dopant related complexes and precipitates, surface and interface states, and dislocations and other extended defects. These defects can arise during the growth process due to incorporation of residual impurities or epitaxial defect formation.
A minority carrier device can degrade during operation for several reasons. In an LED, the carrier injection efficiency or light extraction efficiency can change depending on the particular device structure and the operating conditions. The most common cause of decreased device efficiency is an increase in the non-radiative recombination efficiency caused by the formation of defects in the active region during stress. This process results in the gradual degradation of device characteristics over time, as illustrated by the graph shown in FIG. 4. The graph shows that .eta..sub.external, the external quantum efficiency, decreases as the period of time the device is under stress increases.
A variety of physical processes contribute to the increase in non-radiative recombination centers in the active region during LED operation. Recombination enhanced or photo-enhanced defect reactions within the active region or at nearby edges or interfaces can contribute to the increase. Other processes include the diffusion or propagation of impurities, native point defects, dopants, and dislocations (also known as dark line defects) into the active layer from other regions of the device. These defects and residual or unintentional impurities have always been considered as detrimental to device performance and great efforts have been expended trying to minimize the concentration of these defects and impurities.